To provide a better understanding of the invention it is necessary to distinguish between the two terms analgesia and anesthesia. Analgesia is defined as a condition in which nociceptive stimuli are sensed but are not interpreted as pain. Anesthesia is a state characterized by total loss of sensation, the result of pharmacologic depression of nerve function. Thus, analgesia does not produce anesthesia whereas anesthesia produces analgesia.
In general, pain is associated with a known tissue pathology (e.g., cancer pain, arthritic pain), inflammation, or injury to a body tissue (e.g., surgery). Neuropathic pain is thought to be a consequence of damage to peripheral nerves or to regions of the central nervous system. Neuropathic pain can present as an acute pain but frequently occurs as a form of chronic pain.
The use of long-acting local anesthetics that elicit complete neural blockage for more than several hours is frequently desirable in the management of acute and chronic pain. Pain relief research during the last two decades has focused on the identification of new local anesthetics to produce analgesia of long duration with minimal impairment of autonomic function and low toxicity. One of the best known “long-acting” local anesthetics developed to date, bupivacaine, reportedly blocks major nerve block for three to twelve hours. Unfortunately, bupivacaine is also highly cardiotoxic. The development of alternative “long-acting” local anesthetics met limited success.
Pain relief research also has focused on the identification of new neurolytic agents for the treatment of chronic pain and intractable cancer pain. Historically, spinal opiate administration, surgical intervention, or both have been used to alleviate chronic and intractable cancer pain. When these methods fail to provide sufficient pain relief, phenol or absolute alcohol reportedly have been used as neurolytic agents to destroy the pathogenic nerve regions that are responsible for pain manifestation. However, these agents exert only weak local anesthetic effects and, accordingly, have been difficult to administer to alert patients without inducing additional pain. To date, a long-acting local anesthetic with no major side effects has not been available for the treatment of acute and chronic pain.
In view of the foregoing limitations of the existing local anesthetics to prolong the duration of anesthesia, a need still exists for useful long-acting local anesthetics for pain management. Preferably such local long-acting anesthetics also will exhibit reversible effects. Such drugs would be useful and desirable, for example, in postoperative analgesia, and for treating acute and chronic pain. Preferably, such agents would have sufficient potency to permit administration of a single, relatively low dosage of the agent, thereby minimizing the likelihood of side effects that have been attributed to the existing local long-acting anesthetic agents.
Antidepressants are frequently used as analgesics in pain management but only when administered systemically. Among them, amitriptyline has been used orally for the analgesic therapy of chronic pain. Amitriptyline's sites of action are both central and peripheral. Despite the numerous reports on amitriptyline's analgesic effect on reducing pain when administered systemically, the exact mechanism of this effect remains unknown. To our knowledge, tricyclic and tetracyclic antidepressants have not been used as local analgesics. Likewise, they have not been used at all as anesthetics either locally or systemically.